Algae-resistant shingles have been used for many years and have employed various algae-inhibiting substances to deter the growth of algae on the shingles. A widely used algae-inhibiting substance is copper, and algae-resistant shingles using copper-containing granules are shown and described, for example, in U.S. Pat. No. 5,356,664 to Narayan et al. In that patent, a blend of copper-containing algae-resistant granules and non-algae-resistant granules is used to produce the algae-resistant shingle, with the ratio of copper-containing algae-resistant granules to the non-algae-resistant granules being suggested as 1:9 by weight.
In the Narayan et al. patent, the copper-containing granules comprise a conventional substrate (usually crushed stone) having at least one inner ceramic coating which includes cuprous oxide, and a seal coating which regulates the leach rate of copper ions from the inner ceramic coating. The copper-containing granules leach copper ions during the service life of the algae-resistant shingle to inhibit algae growth on the shingle surface even when the shingle surface is exposed to substantial moisture.
While algae-resistant shingles using copper-containing granules have been well known for many years, they are costly due to the additional cost of the copper-containing granules. As compared with normal ceramic-coated colored granules, copper-containing granules require at least one additional inner ceramic coating comprising cuprous oxide and contain a significant amount of copper-containing raw material. In addition, much of the copper is wasted, since not all of the copper ions are available to be leached from the copper-containing granules. Only the copper ions in the portions of the copper-containing granules exposed to the environment can be leached out onto the shingle surface. The copper ions located in those portions of the copper-containing granules, which are embedded into the asphalt coating of the shingle, are not available to be leached out of the granules, since they are covered by asphalt.